It has been found that the fatigue life of structural joints using fasteners is increased when the fasteners are installed in interference fit, i.e. the diameter of the fastener is larger than the hole diameter so that the holes are expanded to incude a compressive stress gradient therearound during the installation of the fastener. While interference fit has been accomplished using fasteners requiring access to both sides of the workpieces for installation, such interference fit has generally not been available when use of blind rivets was required. It is important that the interference fit be provided along the entire length of the holes including any countersink area where countersunk fasteners are used.
Such blind rivets typically have a relatively soft sleeve mounted on a stem which is pulled to install the rivet. Pulling on the stem usually deforms the sleeve axially to expand the sleeve both inwardly and outwardly to fill the space between the stem and the holes and then forms the blind head on the sleeve. Because the sleeve is soft, it did not have sufficient strength to expand the holes to produce an interference fit. Examples of this type of blind rivet are illustrated in U.S. Pat. Nos. 2,931,532; 3,148,578 and 4,012,984.
A blind rivet which does have the capability of producing an interference fit is proposed in U.S. Pat. No. 3,820,297. This rivet also has a sleeve mounted on a stem to install the rivet by pulling on the stem. The stem has a shank with a diameter larger than the initial inside diameter of the sleeve so that, when the shank is pulled into the sleeve, it expands the sleeve into an interference fit. Since the shank on the stem must be strong enough to expand the sleeve without wire drawing, it is difficult to provide grip adjustment while at the same time permitting the stem to be separated with a breakneck groove so that the end of the stem is always flush with the head of the sleeve. Since the shank on the stem must be at least as long as the grip length of the sleeve and protrudes out of the sleeve for its full length on the blind side of the workpieces prior to being pulled, the required blind side clearance must be greater than that for most other blind fasteners in order for the interference fit blind fastener to be used.
None of these blind rivets has the capability of providing an interference fit in the countersunk area where countersunk blind fasteners were used. Thus, these prior art blind rivets did not have the capability of providing the necessary interference fit to significantly increase fatigue life, especially in the area where the constant diameter holes through the work pieces join with the countersink and where the countersink opened onto the surface of the work pieces. This problem has been appreciated in the past and attempts have been made to prestress the countersink area with solid non-blind rivets which are driven from one side and bucked from the other side of the workpieces during installation. Examples of these type rivets are illustrated in U.S. Pat. Nos. 4,000,680; 4,048,708; and 4,051,592. Because this type rivet requires access to both sides of the work pieces, however, this concept cannot be applied to prior art blind rivets.
Further, application standards for blind rivets require mechanical locking means to lock the stem into the sleeve when the rivet is installed. The prior art has relied on a separate locking collar applied between the head of the sleeve and the stem as an incident to the installation process. This has maintained high manufacturing costs for such blind rivets and/or required more complex tooling to install these rivets.
These prior art rivets have further been installed using a single action, i.e. either pulling or pushing on the stem to move it in a single direction to both expand the sleeve in the hole and form the blind head on the sleeve. As a result, it was not possible to separate the expansion function from the head forming function. Thus, where different force levels are needed during expansion from those needed during blind head formation, installation of these prior art blind rivets to satisfy these criteria have been difficult to achieve.